There are hardly any aquarists who don’t dream of observing "their" fish in the largest aquarium in the world: their natural habitat. In Lake Tanganyika, which holds 18,800 km3 , this is comparatively easy, because it has very clear water and an enormous density of fish. Just by looking under the surface of the water, you’ll be able to see at least ten species found in aquariums. But if you travel to South America and dive into the Rio Negro armed with diving goggles, you will see nothing but tea-brown water. Not even the large freshwater dolphins will come close enough for you to see them underwater. You’ll have to feed them for that.
But with a bit of luck, the seasoned traveller aquarist will find waters in the tropics that allow a certain visibility and then nothing will stand in the way of meeting his or her "favourites". Definitely one of the most common wishes of aquarists in South America is to observe discus, neons, dwarf cichlids, catfish and freshwater stingrays - the Big Five of South America. Apart from the discus, there is a good chance of finding these fish in their natural biotope. Just with the discus is it pure chance, because there are no longer any known places where discus can be found. If there were, discus catchers would be there in a flash. Since neons form really large shoals and can easily be located in the Barcelos region on the Rio Negro, we’ll be able to see them. The real living conditions of the cardinal tetras are exciting: The water contains practically no minerals and resembles distilled water, the pH value of 4.0 is so acidic that not even mosquito larvae survive and the water is extremely brown in colour. There are no plants and hardly any plankton, which is what cardinal tetras feed on. In a plankton net with a diameter of 30 cm, which we dragged behind a boat for 30 minutes, we only found four small plankton organisms!
As soon as the water in the tributaries becomes clearer, you can observe quite a lot of fish species: cichlids, knifefish, tetras and catfish. The water values also change here: the pH value rises towards 7, the hardness is one to three degrees and as soon as iron is measurable in the water, water plants can be found in some places. These is precisely where a look under water is rewarding. All life can be found where the plants are.
A little south of the Amazon basin begins the largest swamp area in the world, the Pantanal. The northern part still has the same species found in the Amazon region, but instead of in murky water, they are in one of the clearest bodies of water in the world. Visibility can easily exceed 50 metres. Here you can accompany rays as they search for food in the sand and watch piranhas as they watch you.
Geophagus species exist above the sands of clear lakes, living up to their name of "eartheater". They dig tirelessly in the fine sand. Tetra enthusiasts will be surprised to learn that Serpae tetras are not schooling fish - they always keep a distance of one metre from the next individual. If they get closer, sparks fly, despite what you may read. Tetras definitely haven’t read it.
Live-bearing toothcarps such as guppies, swordtails and Yucatan mollies are among the most popular aquarium fish of all. Central America (Belize, Mexico, Costa Rica) offers the best chances for seeing these fish species. Aquarists often observe that livebearers also pick at plants. In the biotopes, however, there are hardly any plants. Instead, they behave like growth-eating cichlids from Lake Malawi: they eat algae growing on the substrate with their superior mouth (!). Biologists don't find this funny at all, as it ruins all their theories. Another aspect of the Central American rivers is interesting: The swordtails swim until just before the mouth of the river, where the river water is mixed inseparately with the seawater. But they also swim far back up the river, thrugh water containing fewer and fewer salts until they reach really soft water with an unmeasurable hardness. In aquariums there are huge consequences when we transfer fish from very hard to very soft water. This osmotic shock is caused by the sudden influx of water into the body cells. So why does nothing happen to the fish in the rivers? The answer is simple: swordtails do not swim up the river at 100 km/h – they swim at a very leisurely speed. This gives their bodies plenty of time to get used to the changing salt levels.
No matter which fish you keep in your aquarium, observing these species in their biotope will help you to better understand their requirements and behaviour patterns and enable many a successful breeding that has not so far worked out.